The in situ characterization of small aerosol particles is a persistent objective in many applied contexts. Examples include the determination of atmospheric aerosol composition for climate modeling and the detection of biological or chemical weapons agents for defense applications. Various measurements and calculations of single and multiple-particle scattering patterns are known. The overall goal of such work is to infer information relating to the particles' physical form, such as size and shape, by analyzing the angular structure of these patterns. Unfortunately, a fundamental limitation of this approach is the absence of an unambiguous quantitative relationship between a pattern and the corresponding particle properties, i.e., the so-called inverse problem. Consequently, the inference of these properties from the patterns has proved to be very difficult in practice, except for the simplest of cases. Ideally, one would prefer to image the particles directly, thus eliminating the complexity and ambiguity associated with interpretation of the scattering patterns.
However, the typical particle size range of interest for many applications is roughly 0.1-10 μm. Because of the small size, direct images are possible in only part of this range and only with high numerical-aperture (NA) optics and small focal volumes. Such imaging typically requires the collection and immobilization of particle samples, and thus, is not a practical technique for particle characterization in applications requiring high sample through-put or images of the particles in their undisturbed form, i.e., in situ images.
Holography is an alternative technique that combines useful elements of both conventional imaging and scattering. Fundamentally, this is a two-step process: first, an object is illuminated with coherent light, and then the intensity pattern resulting from the interference of this light with that scattered by the particle is recorded. The resulting pattern constitutes the hologram, from which an image of the object is reconstructed. Traditionally, holograms are recorded with photographic film due to the film's high resolution. Such a high resolution medium is required to capture the finer features of the interference pattern. The subsequent chemical development of the film is costly and time consuming which greatly limits the practical utility of the technique.